MSFT made it clear they wanted Win8 to support everything 7 did. Same NT 6.1 Kernel too, so no real reason to force that issue. I fully expect Win8 to be the last 32bit OS by MSFT though. Which makes me wonder if MSFT will get around to re-coding a LOT of the legacy stuff thats still in Windows...
But for an OS like Linux? No reason to force the issue. Totally different user base.
SPARC is not old architecture it is present.
There is Ultrasparc T2 CPU that is GLP'ed for Microprocessor definition
and is free to develop, manufacture adn enhance in open.: www.opensparc.net
As there are old - x86 CPUs, there is also old SPRCs but that does not say whole architecture is old or something.
It evolves and is being developed further, same for x86 ,SPARC and POWER.